Soil CO2 efflux and root respiration at three sites in a mixed pine and spruce forest: seasonal and diurnal variation

Soil CO2 efflux and respiration of excised roots were measured with a LI-COR 6200 at three sites in a mixed forest (60°05ʹN, 17°3ʹE), from May to October 1999, both day and night. Fine-root (<5 mm in diameter) respiration was measured at ambient root temperature and soil CO2 partial pressure, and the roots were analysed for nitrogen (N) concentration. Root-density data obtained from soil cores were used to estimate fineroot biomass. Coarse-root respiration was estimated using stand data, literature data, and allometric relationships. Soil CO2 efflux, 3.0-7.0 µmol·m-2·s-1, differed between sites but showed no diurnal variation. Maximum values were obtained in July through August. Fine-root respiration, 0.3-4.7 nmol·g-1·s-1, decreased after peaking in early July and showed no diurnal variation. The seasonal mean was lowest at the South site, where also root distribution patterns were different and root N concentrations were lower. Fine-root respiration increased with root N concentration; however, the relationship was very weak, since the variation in root N concentration between sites and times of year was small. Both soil CO2 efflux and fine-root respiration increased exponentially with soil and root temperature, respectively, although fine-root respiration was twice as sensitive. The percentage of soil CO2 efflux emanating from roots was 33-62% in May, thereafter decreasing to 12-16% in October. This, in combination with larger temperature sensitivity for fine-root respiration, is suggested to cause the temperature sensitivity of soil CO2 efflux to diminish over the season.